This invention relates to a method and system for controlling the boundary layer of an airfoil.
Sailplanes generally have laminar flow airfoils. These airfoils allow laminar airflow of up to 70% on the upper surface thereof and up to 93% on the lower surface thereof. Normal transition of laminar flow to turbulent flow occurs through the laminar separation bubble process. It has been shown that a normal laminar separation bubble results in higher profile drag than when transition is forced artificially.
Currently two methods are used to artificially force laminar flow to turbulent flow. The first method utilises a turbulator tape applied to the airfoil, which is provided with small three-dimensional obstacles along its length, and which trips the boundary layer flow from laminar to turbulent. The second method uses pneumatic turbulators which blow air through small holes provided along the length of the lower surface of the airfoil. This again trips the boundary layer flow from laminar to turbulent. The holes are located at a maximum position of 93% of the chord length of the airfoil towards the trailing edge thereof. The second method could also include small holes provided along the length of the upper surface of the airfoil and through which air is sucked into the airfoil.
A conventional airfoil usually includes a movable flap on its trailing edge, which changes the camber of the airfoil and effectively alters the shape of the airfoil. However, the addition of a flap to the trailing edge of the airfoil introduces a flap gap, at the point where the flap moves relative to the airfoil. When the flap is deflected a flap kink forms at the point of deflection. When the flap is in the cruise position (undeflected), air flow over the flap gap is up to 93% laminar flow with the remainder being turbulent flow. Generally, at the flap gap position, the laminar flow is tripped to turbulent flow and only about 84% laminar flow is possible with an associated increase in drag. It is possible, but very difficult, to seal and smooth the flap gap so that the flow is not tripped to turbulent flow at that position.
It has been demonstrated in commercial gliders that it is feasible to take the laminar boundary layer beyond the flap gap and to apply artificial transition on the flap surface. This is, however, only possible in the zero degree flap deflection position, where the bottom surface is undeflected. As soon as the flap is deflected to a positive flap deflection position, the kink in the airfoil surface trips the boundary layer flow to turbulent flow. A laminar separation bubble is then formed at this position and as a result is reattached turbulently on the flap, which leads to increased drag.